Single correct · +4 / −1

For a concave mirror of focal length 10 cm, an object placed at 20 cm gives an image at:

  1. (a) 10 cm (same side)
  2. (b) 20 cm (same side)
  3. (c) 20 cm (opposite side)
  4. (d) infinity
Answer and solution

Answer (b) 20 cm (same side)

Solution

Physics sir · 3 steps

Object at 2f for a concave mirror means the image also forms at 2f, real and in front of the mirror.

  1. Sign convention. Measure distances from the pole, taking the direction of incoming light as positive. The object and the focus of a concave mirror are in front, so u = -20 cm and f = -10 cm.

    1/v = 1/f - 1/u = -1/10 + 1/20 = -1/20

  2. Read the sign. v = -20 cm. The negative sign means the image is in front of the mirror, on the same side as the object, so it is real.
  3. So. The object sits at 2f, the centre of curvature, and the image forms there too: 20 cm on the same side, real, inverted and the same size. That is option (b).

The idea to keep. For a concave mirror, an object at the centre of curvature (2f) gives a real, inverted, same-size image at the same point. Remember it and skip the arithmetic.

Answer from the NTA final answer key. Solution by PrepWiser.

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